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Ball-in-Cage Alarm Switch 


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Ball-in-Cage Alarm Switch 

Written By: Sean Michael Ragan 



Computer (1) 

Drill (1) 

Flat filed) 

Miter box (1) 

Printer (1) 

Rubber cement (1) 

Scissors (1) 

Soldering materials (1) 

Wire bending jig (1) 

Wire cutters (1) 
aka side cutters 

Wire strippers (1) 
or knife 

Dowel (2") 
PVC pipe cap (1) 
Ball bearing (1) 


This clever mechanical switch remains open so long as it is balanced in an upright position. 
But disturb it in any way, and the ball falls against the bars of the cage and closes the 
circuit. Use it to control an alarm to protect your valuables, or to trigger a camera to 

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Ball-in-Cage Alarm Switch 
photograph wildlife. 

Step 1 — Drill out end of dowel 

• Print and cut out a copy of the supplied template. The circle should be 7/8" in diameter. 

• If you can print on to an adhesive label, do that. Otherwise stick the template to a flat-cut 
end of your dowel with rubber cement. 

• Using a 3/32" bit, drill the two gray-tinted holes at least 3/4" deep, and the three other 
holes about 3/8" deep. 

Step 2 — Cut and finish base 




• Using a miter box, saw off the drilled-out end of the dowel 3/4" from the end. 

• Only two of the five holes should penetrate all the way through the 3/4" piece you just cut 

• Optionally, you may may stain or paint the base at this time. A black permanent marker 
worked well for me. 

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Ball-in-Cage Alarm Switch 

Step 3 — Prepare pipe cap 

• Choose a drill bit of the same diameter as your cable. Here it was 1/8". 

• A quick poke with a hot soldering iron is a handy way to make a pilot dimple for your drill 
bit. Just eyeball it as close to the bottom of the cap as you can, allowing for the full 
diameter of the hole you're going to drill. 

• Generally, melting, heating, or burning PVC is a bad idea. There's probably no 
harm in a quarter-second's poke with a soldering iron, but make sure you've got 
proper ventilation anyway, and don't do it at all if you're not aware of and comfortable 
with the risk. 

• Put the cap open-side down on the workbench and drill the hole. 


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Ball-in-Cage Alarm Switch 

Step 4 — Bend cage bars 

• Start with two pieces of 3/32" brass rod, each about 4" long. 

• The bending jig I used was just a handy bit of junk. Any 1/2" diameter bolt or rod should 
work fine. 

• 3/32" brass rod bends easily with the hands. Bend it a bit too far, at first, then open the 
legs back up until they're parallel. 

• Check the spacing of the legs against the holes in the dowel (those on opposite corners of 
the square). 

• If the legs are a bit too wide when parallel, give the bent end of the rod a gentle squeeze 
with a pair of pliers. 

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Ball-in-Cage Alarm Switch 

Step 5 — Cut cage bars to length 

• You're going to make one long and one short "J." 3/32" brass rod can be cut with side- 
cutting pliers and a bit of elbow grease. 

• For the "long" J, cut one leg about 1 1/8" long, measuring from the outside of the bent end 
of the wire. 

• Cut one leg of the "short" J about 7/8" long, measuring the same way. This leg should be 
about 1/4" shorter than the leg on the "long" J. 

• Don't worry about the exact length of the longer leg on each J-they're going to be 
trimmed to length later in the project. 

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Ball-in-Cage Alarm Switch 

Step 6 — Assemble cage 

• First, install the "short" J in the wooden base. Make sure its longer leg is lined up with one 
of the holes that goes all the way through the dowel, and push it in. Its shorter leg goes in 
the "blind" hole on the opposite corner. Go ahead and push it in until it stops. 

• Now insert the longer leg of the "long" J into the other "through" hole. Leave the shorter leg 
clear of its hole, for now. 

• Drop the brass ball into the pocket formed by the three legs that are already inserted in the 

• Rotate the short leg of the long J into position and press it home into the remaining blind 
hole. This action secures the ball in place; it should not be able to escape from the cage 
thus formed. 

• Now is a good time to test the action of your switch. Make sure that the ball can be set in a 
"neutral" position resting in the center hole that doesn't contact any of the bars. Also make 
sure that the circuit closes across the two legs extending through the base when the ball 
falls against the cage. 

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Ball-in-Cage Alarm Switch 

Step 7 — Prepare cable 

• I wanted a round cable for the switch leads to match the round hole drilled in the pipe cap. 
I ended up using a piece of 1/8" coax cut from an old RCA-style video cable. 

• Whatever style of cable you are using, it needs at least two conductors. Strip away the 
cable sheath with wire strippers or a hobby knife and expose the two wires. Strip their 
ends, as well, exposing bare copper. 

• Use a piece of scrap brass rod, or other 3/32" stock, as a mandrel to twist a loop in the 
end of each wire. These loops will fit over the brass pins to form a strong mechanical 
connection before soldering. 

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Ball-in-Cage Alarm Switch 

Step 8 — Assemble connections 



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• Using a small file turned on edge, cut a notch on each side of each brass pin extending 
from the switch base. 

• Notches should be about 1/8" from the bottom of the dowel, and each cut about a third of 
the way through the thickness of the rod. 

• Slip the loops in the ends of the stripped wires over the two brass rods and snug them 
down in the notches you just cut. 

Step 9 — Solder connections 

• As always when soldering, make sure your workspace has proper ventilation and 
be wary of burns. Do not allow flux to contact your eyes or skin. 

• Apply paste flux to each joint and heat with your soldering iron. 

• Touch the solder to the work and wait for it to melt and flow into the joint. 

• Leave the joints to cool, naturally, without disturbance, back to room temperature. 


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Ball-in-Cage Alarm Switch 

Step 10 — Insulate connections 

• Use heat-shrink tubing, if necessary, to make sure the two lead wires cannot accidentally 
short against one another no matter how the switch is turned. 

• If you're using coaxial cable, like I did, a 3/8" length of heat shrink tubing, slipped over the 
cable from the free end, works well to cover the uninsulated exterior conductor from the 
coax pair. 

• I like to use a cigarette lighter to shrink the tubing, but radiant heat from a soldering iron 
will work, too. 

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Ball-in-Cage Alarm Switch 

Step 11 — Trim pins to length 

• Use side-cutting pliers to trim the excess length of the brass pins just past the solder 

• Once cut, each pin should extend about 3/16" of an inch below the bottom of the dowel, 
and no more than 1/4". 

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Ball-in-Cage Alarm Switch 

Step 12 — Final assembly 



• Thread the free end of the cable inside the PVC cap and out through the drilled hole. 

• Now is a good time to perform one more electrical test of the switch's function. 
Once assembled, it will be very difficult to make any repairs. 

• Push the bottom of the dowel into the PVC pipe cap until the top of the dowel is flush with 
its rim. Rubbing the sides of the dowel with a bar of soap makes this a lot easier. 

• If your switch is too hard to balance in the upright open position, file a flat place on 
the bottom of the cap. You can "fine tune" the sensitivity of the switch by filing a 
bigger or smaller flat. 

There are a number of variables that affect the mechanical sensitivity of the switch. The 
diameter of the detent hole drilled in the center of the dowel is an easy one to adjust; larger holes 
will tend to retain the ball longer and will decrease sensitivity. Likewise, the center of gravity of 
the switch assembly can be adjusted by how far the dowel is inserted, or by adding weight to its 
base. Using a PVC cap or a couple that's flat on the end, instead of rounded, will give a rather 
more stable switch. Experiment! 

This document was last generated on 2012-10-31 01:38:35 AM. 

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